The liquid chromatography column is composed of three major parts, that is, a column body tubing, end-fitting blocks at both ends of the column body, and a filler as a static phase(stationary phase) packed in the column.
The column body tubing is made of various material such as stainless steel, glass-lined stainless steel, polymer, and polymer coated silica, etc.
Porous powders are usually used as stationary phases. Porous silica, alumina, zirconia, and their ligand-attached or polymer-coated products are usually used, and various kinds of porous block copolymer powders are also used. The most typical stationary phase at present is porous octadecyl ligand attached silica, C18.
Porous disc-type frits are placed in both ends of a column to keep the stationary phase in and to let the mobile phase penetrate in common columns although they are different in shapes. Such frits are treated very carefully since frits directly influence chromatographic resolution, and various researches for frits have been being carried out.
The monolith column has been recently proposed. The whole stationary phase of the monolith column is one body with numerous multiple porous channels, thus it does not need any frit (Svec et al., Anal. Chem. 1992, 64, 820–822; Minakuchi et al., Anal. Chem., 1996, 68, 3498–3501).
Various types of column end fittings and frit installation mechanisms are illustrated in the “Handbook of HPLC” (1994, GIT-Verlag) edited by Unger. In all cases, frits are installed inside the column main body, and the frits cannot be removed without disassembling the column.
Additional various designs of frits and end fittings have been proposed. For example, polymer encased stainless steel disc frits were used in the U.S. Pat. No. 4,399,032(1983), and trapezoidal disc type frits, in the U.S. Pat. No. 4,966,696(1990). Such frits were used to minimize anomalous perturbation of mobile phase flow and consequent column efficiency degradation owing to differences among the inside diameter of column, the diameter of porous frit disc, and the inside diameter of connecting tubing. Another special design is given in the U.S. Pat. No. 5,227,059(1993) where a tough and flexible polymer insert with a central flow channel was proposed to be installed in front of the column inlet frit in order to remove any void volume owing to prolonged use of the column and consequent partial collapse of the stationary phase by just tightening the insert.
Hernan Cortes et al proposed to prepare an unreplaceable porous ceramic plug frit at the column end by putting a potassium silicate solution into the end of the column main body and by causing sintering on vapor heating in the U.S. Pat. No. 4,793,920(1988). This method was also used in production of microcolumns in the U.S. Pat. No. 5,679,255(1997), and only the treatment of column outlet was mentioned, in particular, but not the treatment of column inlet, which leads us to guess that the conventional separate frit technique was used for the inlet side.
The technique of making fixed frits in the column main body tubing is generally used in production of microcolumns, especially microcolumns made of polymer coated silica capillary.
The initial microcolumn frit designs were, however, nonporous and imperfect, and a metal wire whose diameter was slightly smaller than the inside diameter of the silica capillary, or another silica capillary with very narrow I.D. whose O.D. was slightly smaller than the I.D. of column capillary, was used in the U.S. Pat. No. 4,483,773(1984). In addition, a silica capillary column encased in soft polymer tubing was proposed in the U.S. Pat. No. 5,938,919(1999) where the conventional replaceable frits were used.
The recent trend of silica capillary microcolumns is shifting swiftly toward monolithic columns where the whole column functions as a large frit, too. Details of monolith columns have been introduced in some review articles (Vissers et al., J. Chromatogr. A, 1999, 856, 117–143; Jinno et al., Trends in Analytical Chemistry, 2000, 19, 664–675; Bartle et al, J. Chromatogr. A, 2000, 892, 279–290), and patents of various monolith columns have appeared.
For example, the method of preparing fixed frits at both ends of the column by partial heat treatment of a packed silica capillary was introduced in the literature (Boughfflower et al, Chromatographia, 1995, 40, 329; Smith et al, Chromatogrphia, 1994, 38, 649). A research group made use of such concept, and packed a silica capillary with porous stationary phase powders, and sintered the whole stationary phase by heating with a ring-type electric heating wire, leaving the whole stationary phase combined but the porous spherical structures of the powders unchanged, in the U.S. Pat. No. 5,858,241(1999). A monolith column was proposed to be prepared by putting the dispersed solution of stationary phase powders, water, a solvent such as alcohols, and a metal alkoxide by a syringe in a silica capillary, and by applying heat treatment and vacuum in the U.S. Pat. No. 6,136,187(2000).
The conventional arts of preparing frits of liquid chromatography columns described above can be categorized into two techniques.
The first one is to place separate frits. In this case, the frit is exclusively installed inside the column end-fitting, thus, if the frit is clogged and needs to be replaced, the column end-fitting should be disassembled. Besides, perturbation of mobile phase flow and consequent reduction of column efficiency can occur if the frit diameter is very larger than the column I.D. or the I.D. of connection tubing. Adopting complicated designs for the frit structure to solve such problems yields cost increase and inconvenience of use. Furthermore, it is very difficult to make separate miniaturized frits for microcolumns with I.D. of 0.5 mm or less whose relative importance has been rapidly increased, and, actually, microcolumns with such frits are not commercially available.
The other frit technique is to make permanent frits at the column ends or for the whole column without using separate frits. In this case, column repacking is impossible, and the column lifetime ends when the frit is clogged or a crack or void volume develops in the column. First, in the case of permanent end frits only at the column inlet and outlet, cracks in the packing structure are sometimes observed after formation of end frits, and the column efficiency is degraded. Furthermore, if frit clogging happens in the course of forming the inlet frit after packing the column with the stationary phase powders, then the whole column finishes its lifetime without being used at all.
The monolith column in which the whole stationary phase is one porous phase and a huge frit as well, has raised a lot of interest, and has its own advantages and disadvantages. First of all, it is difficult to make a monolith without a void volume in a rigid conventional stainless steel column with I.D. of 1–5 mm, thus formation of monolith is confined in a heat-shrinkable polymer tubing. Such manufacturing technique seems to suffer from low reproducibility of production. A lot of care should be taken in end fitting treatment of the polymer tubing of low strength, and such products will have endurance problem. It is relatively easy to make silica capillary monolith column with I.D. of 0.5 mm or less, but lots of high level man-power and time are required for preparation of monolith reactants, formation of monolith, and cleaning the monolith. Furthermore, production reproducibility of silica capillary monoliths is not well known, and it is improper to apply to a pilot or industrial scale.
There has been no frit technique so far to make permanent frits not in the column main body but in the connection tubing between the column and other devices such as an injector or detector. If such a frit technique is avaliable, the structure of column end fitting becomes simple, and it is possible to make useful columns at lower prices by using commercial unions without a new special design. The problem of column efficiency reduction owing to a larger frit diameter compared to the column I.D. is eliminated for this tubing/frit design, and the tubing/frit can be easily replaced without disassembling the main column body when the frit is clogged. The problem is, however, it is difficult to make a permanent frit combined to the inner surface of the connection tubing end with endurance over high pressure and mobile phase flow on packing or operating the column.
The inventor studied repeatedly on production of tubing/frit and column of simple design, convenience, and low cost, and developed the method of producing a durable frit/tubing by sintering porous inorganic particles chemically and thermally at the tip of metal connection tubing, and the method of producing a column of a new simple design by installing the frit/tubing at the column outlet union followed by packing the column with a proper stationary phase and finally installing the column inlet union with the frit/tubing of chemically and thermally sintered frit.